This invention relates to an improved coating for thermal emittance of an x-ray tube anode. In particular, the invention discloses a coating which has improved bonding to the x-ray tube anode as well as high thermal emittance.
As stated in U.S. Pat. No. 4,132,916 which is commonly assigned, it is well known that of the total energy involved in an electron beam striking an x-ray target, only 1% of the energy is converted into x-radiation with the remainder of about 99% being converted into heat. As explained in this patent under the "Background of the Invention", it is well known that thermal emittance of x-ray tube anode targets can be enhanced to some extent by coating the target surface outside of the focal spot track with various coating compounds. The emitted heat is radiated through a glass envelope of the x-ray tube and ultimately to the oil circulating in the tube casing.
A variety of thermal emittance enhancing coatings have previously been used. For example, in the above referred to U.S. Pat. No. 4,132,916 there is described a coating composed of zirconium dioxide (ZrO.sub.2), hafnium oxide (HfO), magnesium oxide (MgO), strontium oxide (SrO), cerium dioxide (CeO.sub.2) and lanthanum oxide (La.sub.2 O.sub.3) or mixtures thereof stabilized with calcium oxide (CaO) or yttrium oxide (Y.sub.2 O.sub.3) and mixed with titanium dioxide (TiO.sub.2). This coating provides a "fused" coating on the x-ray anode. While this coating has been commercially acceptable, it has had some problems with low heat emittance. More importantly, the process for applying it to the anode requires stringent parameters. In addition, there have been problems when applying the coating on some alloy substrates and its tendency to "run" onto noncoated areas during required fusing treatment thereby requiring further processing steps. One of the more serious problems has been the fact that during vacuum firing of the coating on the anode, the temperature must be kept below 1400.degree. C. This also limits the user's ability to outgas the anodes prior to tube assembly.
In U.S. Pat. No. 4,029,828 there is described an x-ray tube target coating composed of 80-94% alumina (Al.sub.2 O.sub.3) and 6-20% TiO.sub.2. While this particular coating has good heat emissity, there have been problems with adhesion.
In U.S. Pat. No. 4,090,103 there is disclosed a coating layer composed of molybdenum, tungsten, niobium and/or tantalum metals in combination with a 20-60 volume percent of a ceramic oxide such as TiO.sub.2, Al.sub.2 O.sub.3 and/or ZrO.sub.2. The coatings in this and the '828 patent provide a "non-fused" coating on the x-ray anode which present stability problems under normal operations.